India's purchases of Iranian oil fell 4.1% in September, slipping from August when imports from Tehran hit their highest in at least 15 years, according to ship tracking data and a report compiled by Thomson Reuters Oil Research and Forecasts.

India, Iran's top customer after China, still imported about 552,200 barrels per day (bpd) of oil from the Persian Gulf nation in September, more than double the same month a year ago following the end of sanctions targeting Tehran in January.

India's oil imports from Tehran in January-September grew 90.5% to an average 411,900 barrels per day (bpd) oil, the data showed.

The OPEC producer's total crude oil and condensate sales likely reached around 2.8 million barrels per day (bpd) in September, two sources said, nearly matching a 2011 peak in shipments before sanctions were imposed.

Iran, along with Libya and Nigeria, is allowed to produce "at maximum levels that make sense" as part of any output limits in a surprise deal reached last week by the Organisation of the Petroleum Exporting Countries (OPEC).

Under pressure from sanctions, New Delhi had curbed Iranian oil shipments, reducing Tehran's overall share of India's oil imports to 5.8% in the two fiscal years from April 2013, compared with about 11.3% before sanctions were imposed.

India's average Iranian oil imports in April-September, the first six months of India's financial year, rose nearly four-fifths to about 468,000 barrels per day (bpd), the data showed, with Tehran's share in overall purchases jumping to 11%, just below pre-sanction levels.

Average imports over the period were at their highest in at least 15 years, the data showed.

India, the world's third-biggest oil importer, shipped in a record about 4.47 million barrels per day (bpd) oil in total in September, about 4.4% more than the previous month.

State-run Mangalore Refinery and Petrochemicals was the biggest importer of Iranian oil in September buying about 150,200 barrels per day (bpd), the data showed.

Essar Oil imported about 134,000 barrels per day (bpd) followed by 131,000 barrels per day (bpd) by Indian Oil Corporation and 69,000 bpd by Reliance Industries. Hindustan Petroleum imported about 68,000 barrels per day (bpd) oil.